Method for the production of a feedstock containing usable iron constituents from industrial waste streams

ABSTRACT

A method for the production of an iron-based feedstock suitable for use as the feedstock for steel mills, from industrial waste streams containing iron, by combining an iron poor material such as exhaust fumes from metals production processes with the waste streams, treating the combined waste stream with an ammonium chloride leaching solution, separating the undissolved precipitates comprising iron compounds from the leachant solution, and further treating the undissolved precipitants by elevated temperature roasting, resulting in the iron-based feedstocks.

STATEMENT OF RELATED APPLICATIONS

This application is a continuation-in-part or application Ser. No.08/360,394, filed on Dec. 21, 1994, current pending, which is acontinuation-in-part of application Ser. No. 08/348,446, filed on Dec.2, 1994, now abandoned, which is a continuation-in-part of applicationSer. No. 08/238,250 filed on May 4, 1994, now U.S. Pat. No. 5,464,596,which is a continuation-in-part of application Ser. No. 07/953,645 filedon Sep. 29, 1992, abandoned, which is a continuation-in-part ofapplication Ser. No. 07/820,987 filed on Jan. 15, 1992, which issued asU.S. Pat. No. 5,208,004 on May 4, 1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a process for the recovery ofusable economically valuable products, including a relatively pure ironor direct reduced iron product feedstock and, optionally, an iron oxideand an iron-carbon residual, from industrial waste streams typicallycomprising zinc compounds and iron compounds. A waste materials streamtypically comprising zinc compounds and iron compounds, such as electricarc furnace (EAF) dust, is subjected to a combination of steps includingleaching (digesting), resulting in a precipitate comprising iron oxides(an iron cake or IC), which then is subjected to roasting, resulting inan enriched iron compound (an enriched iron cake or EIC) which can beused as a feedstock for steel mills. The EIC typically is rich in directreduced iron (DRI). Preferably, the precipitate containing iron oxidesis removed from a process for the recovery of zinc oxide and zinc metalfrom industrial waste streams. During the recovery process, carboncompounds can be added to the waste stream, and a cake product isproduced from the undissolved iron and carbon compounds, which also canbe used as a feedstock for steel mills.

The present invention also relates to an enhanced recycling processwhich utilizes iron-rich materials produced by the invention as afeedstock ultimately to a steel mill. The iron-rich materials are fed toa reduction furnace in which the iron-rich cake is reduced to a higherpurity iron product which may be fed a steel mill as the feed. Fumesexhausted from the reduction furnace can be processed by a baghouseor/and by a wet scrubber and the captured materials are then recycled tothe present process where they are used in the recovery process of theinvention. Fumes emanating from the reduction furnace containparticulate matter, and may include potentially valuable zinc, cadmium,and lead constituents. The fumes are filtered in a baghouse, either atthe steel mill's baghouse or at an independent baghouse. The filtercake, which is an iron-poor mixture, may be combined with the initialwaste feed (such as EAF dust) to the present process and/or otheriron-rich materials, and processed according to this invention.

An alternative method of removing the particulate matter from thereduction furnace fumes is the use of a wet scrubber. A primaryembodiment of the alternative recycle of the present invention is topass the reduction furnace fumes through a recirculated water wetscrubber, such as a venturi scrubber. The fume constituents soluble inwater will be removed from the fumes by the recirculated water. Theloaded recirculated water then may be introduced to the ammoniumchloride leach step. Alternatively, the wet scrubber can use an ammoniumchloride solution instead of water. The particulate matter soluble inammonium chloride, such as for example zinc, cadmium, and leadconstituents, will be removed in the ammonium chloride solution in thewet scrubber. The loaded ammonium chloride solution then can be combinedwith the leaching step discussed above, which preferably is an ammoniumchloride leach, resulting in an exceptional increase in the recycle ofwaste streams from, for example, the steel making process.

2. Prior Art

Industrial waste streams typically contain components which haveeconomic value if they can be recovered in an economic fashion. Forexample, U.S. Pat. No. 3,849,121 to Burrows, now expired but which wasassigned to a principal of the assignee of the present invention,discloses a method for the selective recovery of zinc oxide fromindustrial waste. The Burrows method comprises leaching a waste materialwith an ammonium chloride solution at elevated temperatures, separatingiron from solution, treating the solution with zinc metal and coolingthe solution to precipitate zinc oxide. The Burrows patent discloses amethod to take EAF dust which is mainly a mixture of iron and zincoxides and, in a series of steps, to separate out and discard the ironoxides and waste metals, so that the resulting zinc-compound-richsolution can be further treated to recover the zinc compounds.

Waste metal process dust typically has varying amounts of iron, lead,cadmium and other metals, in various forms, contained in the dust. Thefirst step in the Burrows patent is the treating of the EAF dust with anammonium chloride solution to leach any zinc oxide, lead oxide andcadmium oxide present in the dust into solution, without any leaching ofthe iron oxides present. The second step in the Burrows process iscementation in which the solution obtained from the initial leach isfiltered to remove any remaining solids and then zinc dust is added. Thethird step in the Burrows patent then takes the filtrate from thecementation process and cools the filtrate and obtains what are calledzinc oxide crystals. The Burrows patent does not teach the treatment orrecovery of any values from the discarded iron oxide containingprecipitates.

U.S. Pat. No. 4,071,357 to Peters discloses a method for recoveringmetal values which includes a steam distillation step and a calciningstep to precipitate zinc carbonate and to convert the zinc carbonate tozinc oxide, respectively. Peters further discloses the use of a solutioncontaining approximately equal amounts of ammonia and carbon to leachthe flue dust at room temperature, resulting in the extraction of onlyabout half of the zinc in the dust, almost 7% of the iron, less than 5%of the lead, and less than half of the cadmium. However, Peters does notdisclose a method for further treating the removed components notcontaining zinc compounds.

Thus, there exists a need for a method which will allow the recovery ofan iron product from industrial waste streams which can be subjected tofurther treatments, resulting in a relatively pure iron product, such asdirect reduced iron, which can be used as the feedstock for otherprocesses. The industrial waste streams of most interest for thisinvention include a typical electric arc furnace waste stream and theparticulate matter filtered or otherwise removed from various substepsof the invention or the steel mill, particularly from the fumes of areduction furnace, such as for example a rotary hearth furnace.Producing an iron product with a minimum amount of impurities, such aszinc ferrite, is advantageous because the iron product can be used asthe feedstock for steel production processes. A method which results inthe recovery of an iron product has additional value in that the ironproduct can be sold for use in other processes. Furthermore, recoveryand retreatment of exhaust and other waste products from the presentinvention and from other processes and subprocesses has a beneficialeffect on the environment, and a beneficial, economic effect on the costof the steel making process.

Iron is smelted, or refined, in a furnace in which iron ore, coke andlimestone are heated. Scrap iron also can be used as a feed to the ironsmelting furnace. Prior to introducing scrap iron to the furnace, it isde-scaled of iron oxide, or rust. The mill scale, as it is called, is awaste product typically disposed of and not used in the iron productionprocess. Steel is basically an iron alloyed with other chemicalelements. Scrap steel also can be used as a feed in the making of steel.Mill scale also is not used in the steel production process. Finding aneconomical and/or beneficial use for this mill scale would provide ironand steel mills an opportunity to dispose of the mill scale. Likewise,used batteries provide a waste disposal problem. Used batteries also arenot typically used in the steel making process. Rather than disposal ina landfill, it generally is preferable to recycle the used batteries,which are rich in iron oxide. Finding an economical and/or beneficialuse for used batteries would reduce the quantity of such material sentto landfills and provide a recycle for usable components. All of theseiron oxide rich materials can be added to the waste stream feed which isfed into the present process.

As can be seen, there exists a need for a method which will allowexhausts and fumes from reduction furnaces or the like to be filtered ina baghouse or/and a wet scrubber so that the filtrate can be recycledback to the leaching step of the recovery process. This need isaddressed by the present invention.

BRIEF SUMMARY OF THE INVENTION

The present invention satisfies these needs in a method which recovers arelatively pure iron product from a combination of waste materials fromindustrial processes, such as a combination of waste streams fromelectric arc furnaces, typically containing zinc or zinc oxide, andexhaust fumes from reduction furnaces, which typically are iron-poor.The non-iron solids and feed and product solutions used and/or producedin the process can be recycled such that the process has minimal solidor liquid wastes. Other solids can be recovered by treating othercompounds in the waste materials, for example zinc oxide, zinc, metalvalues, and other residues, all of which can be used in other processes.As an alternative embodiment, iron-rich waste products, such as forexample mill scale and used batteries, also can be added to the wastestream feed of the present process.

The preferred iron-poor waste feed stream is taken from fumes emanatingfrom industrial processes. For example, fumes from reduction furnacesand from the iron and steel making processes typically are filtered inbaghouses. Other industrial processes also produce fumes which may befiltered in baghouses. The waste product removed from the fumes in thebaghouses may be subjected to the present process for recovery ofchemical values and production of an iron-rich product. Likewise, thefumes emanating from direct-reduced iron reduction furnaces may befiltered, with the filtrate recycled to the present process.Alternatively, the fumes may be cleaned using a recirculating water orammonium chloride solution wet scrubber. The loaded recirculating wateror ammonium chloride solution (the scrubbant) may be recycled to theammonium chloride leach step of the present invention, as discussedbelow.

The treatment of EAF dust to recover metal values and an iron product isdiscussed in previous patents and patent specifications of the presentinventors and/or their assignees. The present invention produces anenhanced iron-based feedstock created by the addition of iron poor wastematerials to the EAF dust, the combination of which is treated asdisclosed below. Additionally, iron-rich waste materials can be added tothe combination waste stream to help dispose of such iron-rich wastematerials and to produce an iron-based feedstock having an even higherpercentage of iron. The use of mill scale as an iron-enhancer in theiron and steel making processes is contrary to common technology, asmill scale is considered a waste product or impurity. Likewise with usedbatteries. By adding the iron oxide rich material to the EAF dust, andtreating the combined waste material, a resultant iron-rich feedstock isproduced, suitable as a feedstock to the iron and steel makingprocesses.

The alternative iron-rich waste material is common mill scale which isprimarily iron oxides and/or used batteries which contain aneconomically recoverable quantity of iron oxide. Mill scale is the rustwhich is removed from scrap iron or steel before the scrap iron or steelis used as a feedstock for the iron or steel making process,respectively. Mill scale is not used in the iron or steel makingprocess, but, if properly pretreated, is a significant source of ironwhich can be used as a feedstock for the iron and steel making process.Likewise, used batteries also contain a potentially valuable source ofiron oxide. The second waste material typically is a fly ash or fluedust such as EAF containing quantities of recoverable metals, such aszinc, cadmium, lead, copper and/or iron.

The combined waste material is leached with an ammonium chloridesolution resulting in a product solution (leachate) and undissolvedmaterials (precipitate). In the leaching step, the zinc and/or zincoxide dissolves in the ammonium chloride solution along with other metaloxides contained in the waste material, such as lead oxide and cadmiumoxide. The resultant solution is filtered to remove the undissolvedmaterials, including iron oxides and inert materials such as silicates,which will not dissolve in the ammonium chloride solution. The productsolution and the undissolved materials are separated, with both theproduct solution and the undissolved materials being further treated torecover valuable components. For example, the remaining product solutioncan be treated to produce a zinc oxide product of 99% or greater purity.Alternatively, the remaining product solution can be subjected toelectrolysis in which zinc metal plates onto the cathode of theelectrolysis cell. Any remaining product solution after crystallizationor electrolysis can be recycled back to treat incoming waste material.

The undissolved material separated from the product solution is rich iniron oxides, and typically has some impurities such as zinc ferrite. Theundissolved materials can be used without further treatment as afeedstock for steel mills so long as the quantity of impurities is nottoo great. It is preferable, however, to remove the impurities from theiron oxide prior to using the iron oxide as a feedstock. Even morepreferably, reducing the iron oxide to direct reduced iron (DRI) isdesired as DRI can be used to replace part or all of the steel scrapcharge. Many steelmakers purchase DRI produced through other processesoff site to add to the steelmaking process. The present process can becarried out on site and can recover valuable constituents, includingpossibly some wasted iron constituents, normally exhausted from thesteelmaking process.

The iron oxide in the undissolved materials can be reduced to DRI inseveral manners. First, the undissolved materials may be subjected to ahigh temperature roasting step, preferably in the 980° C. to 1315° C.range, to reduce the iron oxide present in the undissolved materials todirect reduced iron. Roasting at this elevated temperature oxidizesand/or drives off the vast majority of the remaining impurities. Toassist in the formation of a more usable direct reduced iron, theundissolved materials can be pelletized with carbon or sodium silicate,or another suitable material, at the end of or after the roasting step.Second, carbon, in the form of activated carbon, carbon dust, carbonpellets or the like, can be introduced to the ammonium chloride andwaste material mixture during the leaching process. Third, carbon can beintroduced to the dried undissolved material cake. When the iron oxideand carbon are heated under a reducing atmosphere, such as CO or CO₂ orother common reducing gases, the carbon will react with the iron oxide,assisting in reducing the iron oxide to DRI. Combining any of thesemethods can result in an even purer direct reduced iron product.

Prior to being leached (digested) by the ammonium chloride solution, thewaste material mixture, which typically includes franklinite andmagnetite, may be preroasted at temperatures greater than 500° C. for apredetermined period of time. The preroasting causes a decomposition ofthe franklinite zinc oxide-iron oxide complex into zinc oxide, ironoxide and other components. The preroasting process generally comprisesthe steps of adding heat to the waste material mixture and/or passingheated reducing gases through the waste material mixture. Although allreducing gases are suitable, hydrogen and carbon-containing gases suchas carbon dioxide are preferred, as well as mixing carbon (activated)with the waste material mixture and preroasting in a gas containingoxygen. While some iron oxide is reduced from Fe₂ O₃ and Fe₃ O₄ to FeO,no elemental iron is produced during the preroasting step. Additionally,iron and iron oxides are not soluble to any degree in the basic ammoniumchloride solution.

The present invention also provides a method by which iron-richby-products produced by the recovery process are reduced in a reductionfurnace which reduces the iron oxide to DRI. Fumes exhausted by thereduction furnace are filtered through a baghouse or/and a wet scrubber.The materials captured by the baghouse or/and wet scrubber may then berecycled back into the leaching step of the recovery process of thepresent invention where they are used in the recovery process. The solidparticles captured by the baghouse may be combined with the primarywaste stream feed, such as EAF dust, or, alternatively, fed as aseparate primary feed to the ammonium chloride leach. The loadedscrubbant liquid from the wet scrubber may be combined with the primaryammonium chloride leachant or, alternatively, if an ammonium chloridesolution is used as the scrubbing liquid, used as the primary ammoniumchloride leachant.

The iron-rich by-products produced by the present process are reducedinto direct-reduced iron (DRI) by heating in an reduction furnace. TheDRI then may be sent to a steel mill where it is used during the steelproduction process. The fumes exhausted by the steel mill, whichtypically are iron-poor, are then filtered through a baghouse, which maybe located at the steel mill, or/and through a wet scrubber, which mayalso be located at the steel mill. The materials captured by thebaghouse or/and wet scrubber may then be recycled back to the presentrecovery process.

Additionally or alternatively, fumes exhausted from the reductionfurnace used to reduce the iron-rich materials into DRI are fed to abaghouse or/and a wet scrubber containing a heated ammonium chloridesolution. The fumes, which typically are iron poor, consist primarily ofzinc, lead, and cadmium. The materials captured through the baghouse orwet scrubber filtering process are then recycled back into the leachingstep of the recovery process of the present invention.

If the fumes are filtered through a baghouse, the captured materialswill be solids which are placed into the waste material stream wherebythey are added to the ammonium chloride solution of the leaching step.If the fumes are filtered through a wet scrubber, the captured materialswill be discharged from the wet scrubber in a liquid stream directlyinto the ammonium chloride solution of the leaching step. Alternatively,if ammonium chloride is used as the scrubbing liquid, the ammoniumchloride scrubbant may be used as the leaching (digesting) solution. Ifthe scrubbant becomes too loaded, make-up ammonium chloride can beadded. If the wet scrubber uses an ammonium chloride solution as thescrubbing liquid, it should be maintained at approximately 90° C. and23% by weight ammonium chloride in water.

Therefore, it is an object of the present invention is to provide awaste material recovery process which recovers chemical values fromindustrial waste streams, recycles exhaust fumes from furnaces such aselectric arc furnaces and reduction furnaces, recycles exhaust fumesfrom industrial processes such as iron and steel making processes, andrecycles other waste materials, including both iron-rich and iron-poorwaste materials, to produce valuable products.

Another object of the present invention is to provide a process whichuses the waste streams of various industrial processes, particularly theiron and steel making processes, so as to achieve an economical,environmentally friendly recycle process in the steel making industry.

Another object of the present invention is to provide a method forrecovering iron and iron oxide from waste materials, such as exhaustfumes, furnace fumes, mill scale, used batteries, fly ash or flue dust,which contain other metals, such as zinc, lead oxide, and cadmium.

Another object of the present invention is to provide a method forrecovering iron oxide which can be used as is as a feedstock for steelproduction processes.

Another object of the present invention is to provide a method forproducing direct reduced iron from iron oxide recovered as a residuefrom an ammonium chloride leached waste material, such as exhaust fumes,furnace fumes, mill scale, used batteries, fly ash or flue dust.

Another object of the present invention is to provide a method forrecovering an iron product such as direct reduced iron and/or iron oxidewhich is economical, quick and efficient.

Another object of the invention is to provide a method for processingiron-rich by-products produced by the recovery process of the presentinvention to capture materials which can be recycled back into theleaching step of the recovery process of the present invention.

These objects and other objects, features and advantages of the presentinvention will become apparent to one skilled in the art after readingthe following Detailed Description of a Preferred Embodiment.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic of a representative process of the presentinvention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

By taking an iron cake comprising for the most part iron oxides, androasting it at elevated temperatures under a reducing atmosphere, aproduct can be made which is equivalent to direct reduced iron. Ingeneral terms, heating the iron cake above 980° C. up to about 1260° C.,and typically no higher than 1315° C., a direct reduced iron product isformed. This direct reduced iron product then can be pelletized withcarbon or with a sodium silicate, or other suitable compound, after itcomes out of the furnace. The final product then can be used as afeedstock for steel mills without any additional treatment.

As discussed below, the additional step of roasting the iron cake, whichis the undissolved precipitate, to reduce the iron oxide and to driveoff any zinc, cadmium, and lead, and other impurities, is added to theend of a zinc oxide recovery process. The resulting iron product mayhave been reduced from several forms of the iron, such as FeO, Fe₂ O₃,or Fe₃ O₄, reduced to an iron extremely usable as the feedstock forsteel mills.

The waste material, such as the combination of iron poor materials froma baghouse or wet scrubber and EAF flue dust, is leached using ammoniumchloride, and the remaining undissolved precipitate is, for the mostpart, an iron oxide cake. Iron-rich materials also may be added to beleached and further processed. During the roasting of the undissolvedprecipitate, the bond to the non-leachable zinc oxide-iron oxidecomplex, franklinite, contained in the undissolved precipitate isbroken, and the zinc oxide compounds are exhausted in the off gas andcaptured in a pollution control device, such as a baghouse, leaving theiron oxide cake as the residue. The iron oxide cake is roasted at anelevated temperature, causing the reduction of the iron oxide, leavingthe iron metal values. The iron then can be mixed with a binder andformed into briquettes or cubes to be used as the feedstock. Theexhausted impurities then can be recycled to recover, for example, zincoxide, cadmium metal, and lead metal.

The method for recovering an iron product feedstock disclosed herein iscarried out in its best mode in recovering the waste material from thewaste streams of industrial or other processes, and combining it withwaste material recovered from furnace exhaust streams. Many processesproduce an iron poor waste stream, such as reduction furnaces and ironand steel making processes. Many other processes produce an iron oxiderich waste stream. Other processes remove iron oxide rich materialsprior to processing. The iron poor materials are combined with a typicalindustrial waste stream which, after treatment, results in an iron-richmaterial suitable for use as a feedstock to a steel mill. Iron oxiderich materials also can be combined with the typical industrial wastestream and the iron poor waste stream. A typical industrial waste streamused is a flue gas where the charge contains galvanized steel, havingthe following percent composition:

                  TABLE I                                                         ______________________________________                                        Analysis of Flue Dust                                                         Component       Percent By Weight                                             ______________________________________                                        Zinc Oxide      39.64                                                         Iron Oxide      36.74                                                         Lead Oxide      5.72                                                          Inert Materials 9.10                                                          Calcium Oxide   2.80                                                          Potassium Oxide 2.41                                                          Manganese Oxide 1.29                                                          Tin Oxide       1.13                                                          Aluminum Oxide  0.38                                                          Magnesium Oxide 0.33                                                          Chromium Oxide  0.16                                                          Copper Oxide    0.06                                                          Silver          0.05                                                          Unidentified Materials                                                                        0.22                                                          ______________________________________                                    

General Process Description

Generally, the present process is a continuous method for the recoveryof an iron product feedstock from waste material streams. The basicprocess steps comprise:

Basic Process Steps

a. combining a typical industrial process waste material stream, such asfrom a metal or metal product process, with an iron poor waste material,such as from a reduction furnace or the iron and steel making processes;

b. treating the waste material combination with an ammonium chloridesolution at an elevated temperature to form a product solution and anundissolved precipitate comprising iron oxide;

c. separating the product solution from the undissolved precipitatecomprising the iron oxide; and

d. further treating the undissolved precipitate in a roasting processresulting in the recovery of a relatively pure iron product.

The iron poor waste material, if in solid form such as from a baghouse,is added to Basic Process Step a. Alternatively, the iron poor wastematerial, if in solution form such as from a wet scrubber, is added toBasic Process Step b.

To the basic process steps, a number of additional steps may be addeddepending on the process conditions and iron properties desired. Theadditional steps include, either individually or in some combination:

1. preroasting the solid waste material at an elevated temperature;

2. preroasting the solid waste material at an elevated temperature andin a reducing atmosphere;

3. pretreating the solid waste material with an ammonium chloridesolution at an elevated temperature to form a product solution and anundissolved precipitate comprising iron oxide, separating the productsolution from the undissolved precipitate, roasting the undissolvedprecipitate at an elevated temperature and optionally in a reducingatmosphere, and then treating the undissolved precipitate with anammonium chloride solution at an elevated temperature to form a productsolution and an undissolved precipitate comprising iron oxide, andseparating the product solution from the undissolved precipitate; and/or

4. preroasting the solid waste material at an elevated temperature andoptionally in a reducing atmosphere, pretreating the waste material withan ammonium chloride solution at an elevated temperature to form aproduct solution and an undissolved precipitate comprising iron oxide,separating the product solution from the undissolved precipitate,roasting the undissolved precipitate at an elevated temperature andoptionally in a reducing atmosphere, and then treating the undissolvedprecipitate with an ammonium chloride solution at an elevatedtemperature to form a product solution and an undissolved precipitatecomprising iron oxide, and separating the product solution from theundissolved precipitate.

To the basic process steps, additional iron product purification stepsmay be added. For example:

1. elemental carbon can be added during the leaching step or steps toinitiate the reduction of the iron oxide into direct reduced iron duringthe leaching step or steps. The elemental carbon may be added in anumber of forms including, but not limited to, dust, granules, andpellets. The elemental carbon does not go into solution and remains withthe undissolved precipitate.

2. elemental carbon can be added to the undissolved precipitate after ithas been separated from the product solution. Combining elemental carbonand iron oxide in this manner at an elevated temperature and under areducing atmosphere also will initiate the reduction of the iron oxideinto direct reduced iron. The elemental carbon can be mixed into theundissolved precipitate in a number of manners including, but notlimited to ribbon blenders and mixers.

Preferred Embodiment

Referring to FIG. 1, a preferred embodiment of the process is shown.Subprocess 100, the digestion and filtration steps, generally comprisesthe process disclosed and claimed in parent application Ser. No.08/238,250, which also is disclosed above. Subprocess 200, the directreduced iron production steps, generally comprises the process disclosedand claimed in parent application Ser. No. 08/348,446, which also isdisclosed above. Subprocess 300, the chemical values recovery steps,when combined with subprocess 100, generally comprises the processdisclosed and claimed in parent application Ser. No. 08/302,179, whichalso is disclosed above. Subprocess 400, the enhanced direct reducediron production steps, when combined with subprocess 200, generallycomprises the process disclosed and claimed in parent application Ser.No. 08/360,394, which also is disclosed above.

Subprocess 500 comprises the feed process to the present invention. Feedstreams such as iron poor waste fume streams from electric arc furnaces12 and other furnaces such as reduction furnaces or smelters 14 arefiltered in a baghouse 16. Other feed streams such as iron rich directreduced iron and pig iron, as well as scrap iron and steel, aresubjected to the iron or steel making process. Exhaust fumes from suchprocesses, which typically include an electric arc furnace or otherreduction furnace, also are filtered in a baghouse 16. The constituentsfiltered out in baghouse 16 comprise the waste stream feed to subprocess100.

In subprocess 100, the waste stream feed is leached in digester 18 withammonium chloride, preferably at approximately 90° C. and approximately23% by weight concentration. Constituents soluble in ammonium chloridego into solution, while constituents insoluble in ammonium chloride,such as iron oxides, precipitate out. The precipitates are filtered fromthe solution in filter 20. The filtered solution is sent to cementer 22,and subjected to subprocess 300 to recover other chemical values. Theprecipitate, which is an iron cake (IC), is sent to subprocess 200.

In subprocess 200, the precipitate is dried and crushed in dryer/crusher24. Exhaust gases from dryer/crusher 24 may be sent to a baghouse suchas baghouse 16, but more typically are sent to an air scrubber such asair scrubber 26 for cleaning, as the exhaust gases from dryer/crusher 24typically do not have a significant quantity of recoverableconstituents. The dried and crushed precipitates are compacted incompactor 28 and sent to a reduction furnace or smelter 14. In reductionfurnace 14, the dried and crushed iron cake is roasted at between 980°C. and 1315° C., producing an enriched iron cake (EIC) which cancomprise direct reduced iron (DRI) or pig iron, which can be in liquidform. The EIC can be compacted in a second compactor 30, and then cooledby cooling water in a cooling conveyor 32, to produce the DRI. The DRIthen can be used as the feed to a steel mill EAF, and the process cyclestarts over.

Exhaust fumes from the reduction furnace 14 are sent to scrubber 34,which preferably is a recirculating wet scrubber using water or anaqueous ammonium chloride solution. Exhaust fumes from EAFs such as EAF12 also can be sent to scrubber 34. In scrubber 34, the exhaust fumesare scrubbed and the scrubbed off-gas released. The water or aqueousammonium chloride solution containing the constituents scrubbed from theexhaust fumes is sent either to cementer 22 or digester 18, depending onpurity; more pure solutions typically are sent to digester 18, whileless pure solutions typically are sent to cementer 22.

In the preferred embodiment, the furnace 12, 14 off-gases comprise ZnOand other particulate impurities. If the off-gases are scrubbed inscrubber 34, the water balance is maintained using a temperature controlsuch as heat exchanger 36. Additionally, the concentration of ZnO andother solubles in the scrubbing liquid may be controlled by the additionof water W to the cementer 22, or ammonium chloride to the scrubber 34.As discussed above, if an ammonium chloride solution is used as thescrubbing liquid, it is preferred to maintain the solution atapproximately 90° C. and approximately 23% NH₄ Cl.

Preroasting Process

The preroasting step, as mentioned above, can be carried out prior tothe initial leaching step, or between a first and second leaching step,or both. The powder containing the franklinite and magnetite, such asthe waste dust or the combination of waste dust and the iron oxide richmaterial, is heated to temperatures greater than 500° C. Thistemperature causes a reaction which causes a decomposition of the stablefranklinite phase into zinc oxide and other components, and yet does notallow for the complete reduction of zinc oxide to zinc metal. Theresulting zinc oxide can be removed by sublimation or extraction with anammonium chloride solution, such as by following the steps detailedabove under the general process. The resulting material after extractionhas less than 1% by weight zinc.

The solid waste material can be preroasted using many conventionalroasting processes, such as, for example, direct or indirect heating andthe passing of hot gases through the dust. For example, non-explosivemixtures of reducing gases, such as hydrogen gas and nitrogen or carbondioxide, can be passed through the powder containing franklinite andmagnetite. Hydrogen gas is not the only species that may be used forreductive decomposition of franklinite. It is possible to use carbon orsimple carbon containing species, including carbon-containing reducinggases and elemental carbon. Heterogeneous gas phase reductions arefaster than solid state reductions at lower temperatures and thereforesuggest the use of carbon monoxide. The carbon monoxide can be generatedin situ by mixing the franklinite powder with carbon and heating in thepresence of oxygen at elevated temperatures. The oxygen concentration iscontrolled to optimize CO production. The carbon monoxide may beintroduced as a separate source to more clearly separate the rate ofcarbon monoxide preparation from the rate of Franklinite decomposition.The prepared zinc oxide then can be removed by either ammonium chlorideextraction or sublimation.

The optional iron oxide rich material may be added to the process eitherbefore or after this preroasting step. As the preroasting step mainly isto assist in the decomposition of franklinite, if the iron oxide richmaterial is devoid of franklinite, it need not be subjected topreroasting.

Leaching Treatment

The waste material is subjected to an ammonium chloride leach. Anammonium chloride solution in water is prepared in known quantities andconcentrations. If the two-stage leaching process is used, the feedmaterial, such as the waste material flue dust described in Table Icombined with any other feed material source which contains iron oxide,is added to the ammonium chloride solution. Otherwise, the feed materialfirst is roasted. The majority of the waste material mixture, includingany zinc and/or zinc oxide, lead oxide, cadmium oxide, and other metaloxides, dissolves in the ammonium chloride solution. The iron oxide doesnot dissolve in the ammonium chloride solution. As an example, thesolubility of zinc oxide in ammonium chloride solutions is shown inTable II.

                  TABLE II                                                        ______________________________________                                        Solubility of ZnO in 23% NH.sub.4 Cl solution                                                g Dissolved/100 g                                              Temperature °C.                                                                       H.sub.2 O                                                      ______________________________________                                        90             14.6                                                           80             13.3                                                           70             8.4                                                            60             5.0                                                            50             3.7                                                            40             2.3                                                            ______________________________________                                    

It has been found that a 23% by weight ammonium chloride solution inwater at a temperature of at least 90° C. provides the best solubilityfor a waste stream comprising a significant quantity of zinc oxide.Concentrations of ammonium chloride below about 23% do not dissolve themaximum amount of zinc oxide from the waste material, and concentrationsof ammonium chloride above about 23% tend to precipitate out ammoniumchloride along with the zinc oxide when the solution is cooled.Therefore, 23% has been chosen as the preferred ammonium chloridesolution concentration. The iron oxide and inert materials such assilicates will not dissolve in the preferred solution.

The zinc oxide, as well as smaller concentrations of lead or cadmiumoxide, are removed from the waste material by the dissolution in theammonium chloride solution. The solid remaining after this leaching stepcontains iron oxides and some impurities including zinc, lead, cadmium,and possibly some other impurities. The remaining solid then can beroasted in a reducing atmosphere, typically at a temperature greaterthan 420° C. and often at 700° C. to 900° C. The reducing atmosphere canbe created by using hydrogen gas, simple carbon species gases such ascarbon dioxide, or by heating the material in an oxygen containing gasin the presence of elemental carbon. The carbon preferably is in theform of dust or pellets. Typical preroasting times are from 30 minutesto 4 hours. As discussed above, the waste material first may bepreroasted and second may be leached, omitting the first leaching step.

If the iron poor material is removed from the industrial waste streamusing a wet scrubber, the preferred wet scrubber is an ammonium chloridesolution wet scrubber. By using an ammonium chloride wet scrubber, theloaded scrubbing solution, ammonium chloride, can be combined directlywith the ammonium chloride leachant, or sent directly to the cementationstep for removal of certain non-iron products. Alternatively, the loadedammonium chloride scrubbing solution may act as the primary leachant inBasic Process Step b. Depending on the degree of loadedness of theammonium chloride scrubbing solution, pure make-up ammonium chloridesolution can be added to increase the effectiveness of the ammoniumchloride leachant.

As another alternate embodiment, iron-poor waste streams in both solidand liquid form may be added to the present process for treatment. Thesolid stream may be added to Basic Process Step a, while the liquidstream may be added to Basic Process Step b.

Optional Carbon Addition Step

The present process also can be operated to produce a high-qualityiron-carbon cake as a residual product. The iron oxide contained in thewaste stream does not go into solution in the ammonium chloridesolution, but is filtered from the product solution as undissolvedmaterial. This iron oxide cake can be used as is as the feedstock to asteel mill; however, as previously discussed, it becomes more valuableif reduced by reaction with elemental carbon to produce an iron-carbonor direct-reduced iron product. One preferred method for producing suchan iron-carbon or direct-reduced iron product from the waste materialcomprises the steps of:

a. treating the waste material combination with an ammonium chloridesolution at an elevated temperature to form a product solution whichcomprises dissolved zinc and dissolved zinc oxide whereby any iron oxidein the waste material will not go into solution;

b. adding carbon to the product solution whereby the carbon will not gointo solution; and then

c. separating the product solution from the undissolved materialspresent in the product solution including any of the iron oxide and thecarbon.

A mixture of iron oxide and carbon is used by the steel industry as afeedstock for electric arc furnaces. The iron oxide cake which isremoved as undissolved material from the leaching step is primarily ironoxide, being a mixture of Fe₂ O₃ and Fe₃ O₄. The iron oxide cake can bemade into the mixture of iron oxide and carbon by adding elementalcarbon to the iron oxide cake in several manners. First, carbon can beadded to the leaching tank at the end of the leaching step but beforethe undissolved materials are separated from the product solution. Sincethe carbon is not soluble in the ammonium chloride solution and will notreact in an aqueous solution, the iron oxide cake and the carbon can beseparated from the product solution and made into a hard cake. Differentsize carbon, such as dust, granules, or pellets, may be used dependingon the desires of the steel makers. Second, the carbon can be added tothe iron oxide after the iron oxide has been separated from the productsolution. The dried iron oxide and the carbon can be ribbon blended in aseparate process.

Combining carbon and iron oxide in a reducing atmosphere and at anelevated temperature results in the reduction of the iron oxide,producing direct-reduced iron (DRI). DRI can be used to replace part orall of the steel scrap charged to a steel mill. In some operations, DRIis preferred to scrap because it has a known uniform composition andgenerally contains no residual elements such as chromium, copper,nickel, and tin. When carbon-enriched iron oxide is melted, it forms adesired foamy slag because it contains both carbon and iron oxide.Because the price of steel scrap usually is lower than DRI, the use ofDRI usually cannot be economically justified. DRI typically runs in the$120.00 and higher per ton range. However, since the iron oxide is aresidual product of an economical recovery process, such as the recoveryof zinc oxide from flue dust described generally below, with the mainvalue of the process being from the zinc oxide product, the iron oxideor direct-reduced iron can be produced more economically. Therefore, theiron oxide produced as a residual in this process has significant value.

Generally the iron oxide and carbon product is pressed into a cake forease of handling and use. The cake typically contains approximately 82%solids, but may range from 78% to 86% solids and be easily handled andused. Although cakes of less than 78% solids can be formed, the other22%+ of material would be product solution which, if the cake is used asa feedstock to a steel mill, would be reintroduced to the steel-makingprocess, which is uneconomical. Likewise, drying the cake to have morethan 86% solids can be uneconomical.

The roasting process produces vapors, from the zinc, lead and cadmiumand other impurities, that have to be condensed into dust. Theseimpurities can be sent to the baghouse at the end of the steel makingprocess, mixed into the original waste dust, and then sent to the firstleaching step, in a recycle fashion. Alternatively, the exhaust vaporsand dust from the roasting step may be sent to a separate baghouse at astand alone facility.

There are two preferred ways to add carbon to the iron oxide cake.First, it may be beneficial when the iron oxide cake comes out of thereclamation process to grind up the iron oxide cake, pelletize it withcarbon and put it in a roasting furnace. Second, carbon can be added tothe furnace with the iron oxide.

The iron oxide cake can be treated in three manners. First, carbon canbe added to the leaching step and the iron oxide cake will have carbonplus iron oxide. The iron oxide-carbon cake can go directly to the steelmill and, if it goes directly to the steel mill, then the reduction ofthe iron oxide would take place in the steel mill furnace. Second, theiron oxide-carbon cake can be pelletized and roasted in a reductionfurnace to form direct reduced iron. The iron oxide precipitate, whichtypically contains around 80% solids, is ground up with carbon andformed into pellets, briquettes or cubes and then heated. These pellets,briquettes or cubes then can be introduced to a steel making furnace.The difference in the material that would be introduced to the furnacefrom the first manner and the second manner is that in the secondmanner, direct reduced iron is introduced to the steel making furnace,while in the first manner, a combination of iron oxide and carbon isintroduced to the steel making furnace. The iron oxide plus carbon canbe supplied to the steel mill as is. When this carbon enriched ironoxide is melted, it forms a foamy slag, and a foamy slag is desirable insteel making. Third, the carbon can be added through a ribbon blender,and then the iron oxide-carbon cake can be introduced either directlyinto the furnace or, preferably roasted in a reduction furnace first toform direct reduced iron, which would be preferred for steel making.

In order of preference, the first manner is the least preferable, thatis adding the material itself as a mixture of carbon and iron oxidewithout any reducing agents mixed in with it. The second most preferableis the third manner, adding the material with carbon added to it eitherthrough the leaching step or through a ribbon blender and put directlyinto the furnace. The most preferable is the second manner, where carbonis added either though the leaching step or a ribbon blender,pelletizing or briquetting it, roasting it, and introducing it to thesteel furnace.

In any manner, the fumes exhausting from the steel mill furnace and thereduction furnace typically are iron poor, but comprise other valuablecomponents. The furnace exhaust fumes are an excellent source of ironpoor waste materials useful for recovery in the present process. Theexhaust fumes may be filtered in a baghouse, with the resulting filtratebeing added to the waste stream fed of the present process, or with theresulting filtrate being the primary waste stream feed of the presentprocess. The exhaust fumes also may be scrubbed in a wet scrubber, withthe resulting loaded scrubbing solution being added to the ammoniumchloride leachant of the present process. If an ammonium chloridescrubbing solution is used instead of water, the loaded ammoniumchloride scrubbing solution may be used as the primary leachant of thepresent process.

EXAMPLES

The following Examples are taken from data demonstrating ways toincrease the formation of zinc oxide from the product solution producedaccording to the present invention. Examples 1 and 2 do not includeroasting and Examples 3 and 4 include roasting. The Examples areintended to illustrate the initial treatment of the waste materialstream, including the leaching treatment and the initial waste materialstream roasting, if employed.

Example 1 Prior Art

A metal dust of composition listed in Table I of the Burrows patent isadded to 23% by weight NH₄ Cl solution (30 g NH₄ Cl per 100 g H₂ O), asdiscussed in the Burrows patent, in the amount of 1 gram of dust per 10grams of solution. The solution is heated to a temperature of 90° C. andstirred for a period of 1 hour, during which the zinc oxide in the dustdissolves. The remaining solid, which has a composition of approximately60% iron oxide, 5% calcium oxide, 5% manganese, 30% other materials, isfiltered out of the solution. This remaining solid is further treatedaccording to the invention to recover feedstock grade direct reducediron or iron oxide.

Example 2

A metal dust of composition listed in Table I is added to 23% weight NH₄Cl solution (30 g NH₄ Cl per 100 g H₂ O). 1 gram of dust is used per 10grams of solution. The solution is heated to a temperature of 90° C. andstirred for a period of 1 hour. During this period the zinc oxide in thedust dissolves. The remaining solid, having a composition ofapproximately 60% iron oxide, 5% calcium oxide, 5% manganese, 30% othermaterials, is filtered out of the solution. This remaining solid isfurther treated according to the invention to recover feedstock gradedirect reduced iron or iron oxide.

Example 3

A dust containing 19.63% Zn, 27.75% Fe, 1.31% Pb, 9.99% Ca, and 0.024%Cd (analysis based on elements not oxides) was leached at 100° C. in a23% ammonium chloride solution. The solid remaining after the leachingprocess was dried and analyzed to contain 12.67% Zn, 4.6% Ca, 35.23% Fe,0.7% Pb, and 0.01% Cd. This material was placed in a quartz boat in thepresence of activated carbon and heated at 900° C. for two hours in anatmosphere of 95% N₂ and 5% O₂. After two hours, the material wasremoved and added to a 23% ammonium chloride solution at 100° C. Thematerial was filtered and dried at 140° C. for one hour to determine itscomposition. Analysis of this remaining solid was 42.84% Fe, 0.28% Zn,<0.1% Pb, and <0.01% Cd. This remaining solid is further treatedaccording to the invention to recover feedstock grade direct reducediron or iron oxide.

Example 4

A dust with the composition given in Table I is leached in 23% ammoniumchloride solution for 1 hour at 100° C. The solid remaining (whichcontained 14% Zn) was placed in a quartz boat and heated to 700° C. inan atmosphere of 8% H₂ and 92% Ar. The material was cooled and reheatedat 100° C. in 23% ammonium chloride solution at 100° C. The solid wasseparated, dried and analyzed for zinc. The zinc was found to be lessthan 1%. The leached-roasted-leached material then can be subjected tothe remainder of the general process.

Optional Recovery of Zinc Oxide From Product Solution

To recover the zinc oxide from the product solution in subprocess 300,while the filtered zinc oxide and ammonium chloride solution is still ata temperature of 90° C. or above, finely powdered zinc metal is added tothe solution. Through an electrochemical reaction, any lead metal andcadmium in solution plates out onto the surfaces of the zinc metalparticles. The addition of sufficient powdered zinc metal results in theremoval of virtually all of the lead of the solution. The solution thenis filtered to remove the solid lead, zinc and cadmium.

Powdered zinc metal alone may be added to the zinc oxide and ammoniumchloride solution in order to remove the solid lead and cadmium.However, the zinc powder typically aggregates to form large clumps inthe solution which sink to the bottom of the vessel. Rapid agitationtypically will not prevent this aggregation from occurring. To keep thezinc powder suspended in the zinc oxide and ammonium chloride solution,any one of a number of water soluble polymers which act asantifiocculants or dispersants may be used. In addition, a number ofsurface active materials also will act to keep the zinc powdersuspended, as will many compounds used in scale control. These materialsonly need be present in concentrations of 10-1000 ppm. Various suitablematerials include water soluble polymer dispersants, scale controllers,and surfactants, such as lignosulfonates, polyphosphates, polyacrylates,polymethacrylates, maleic anhydride copolymers, polymaleic anhydride,phosphate esters and phosponates. Flocon 100 and other members of theFlocon series of maleic-based acrylic oligomers of various molecularweights of water soluble polymers, produced by FMC Corporation, also areeffective. Adding the dispersants to a very high ionic strength solutioncontaining a wide variety of ionic species is anathema to standardpractice as dispersants often are not soluble in such high ionicstrength solutions.

At this stage there is a filtrate rich in zinc compounds and aprecipitate of lead, cadmium and other products. The filtrate andprecipitate are separated, with the precipitate being further treated,if desired, to capture chemical values. The filtrate may be treated inseveral maimers, two of which are preferred. First, the filtrate may becooled resulting in the crystallization and recovery of zinc oxide.Second, the filtrate may be subjected to electrolysis resulting in thegeneration and recovery of metallic zinc.

To recover zinc oxide, the filtrate then is cooled to a temperature ofbetween about 20° C. and 60° C. resulting in the crystallization of amixture of zinc compounds. The mixture contains a significant amount ofdiamino zinc dichloride, or other complex compounds which involves zincamino complexes, hydrated zinc oxides and hydroxide species.Crystallization helps to achieve a high purity zinc oxide of controlledparticle size, typically through control of the temperature-time coolingprofile. Reverse natural cooling, that is cooling the solution slower atthe beginning of the cooling period and faster at the end of the coolingperiod, is preferred to control the nucleation to crystal growth ratioand, ultimately, the crystal size distribution. The precipitatedcrystallized solid is filtered from the solution and washed with waterat a temperature of between about 25° C. and 100° C. The filteredsolution is recycled for further charging with feed material. Thediamino zinc dichloride dissolves in water. The solubility of diaminozinc dichloride in water is shown in Table III.

                  TABLE III                                                       ______________________________________                                        Solubility of Zn(NH.sub.3).sub.2 Cl.sub.2 in water                            Temperature °C.                                                                      g Dissolved/100 g H.sub.2 O                                     ______________________________________                                        90            32                                                              80            24                                                              40            21                                                              25            12.8                                                            ______________________________________                                    

Very little of the hydrated zinc oxide dissolves in the water. Thisresultant solution then is filtered to remove the hydrated zinc oxidespecies. The solid hydrated zinc oxide species filtered from thesolution is placed in a drying oven at a temperature of over 100° C.After a sufficient drying period, the resultant dry white powder isessentially pure zinc oxide. The filtrate from the solution is recycledfor charging with additional zinc compound mixture.

The zinc oxide may be dried at approximately 100° C. To ensure that thematerial is free of chloride, however, it is preferable to heat the zincoxide to a higher temperature. Diamino zinc dichloride decomposes at271° C. and ammonium chloride sublimes at 340° C. Therefore, heating thezinc oxide to a temperature above 271° C. is useful. The dryingtemperature should be kept below approximately 350° C. to prevent thesublimation of significant amount of ammonium chloride. Therefore, it ispreferable to dry the zinc oxide at a temperature in the range of 271°C. to 350° C. Typically, the zinc oxide should be dried in thistemperature range for approximately 2 to 60 minutes, and preferably from5 to 20 minutes. A 10 minute drying time has been found to be asatisfactory average.

As the zinc, lead and cadmium contained in the feed materials areamphoteric species, by using ammonium chloride solution these specieswill go into solution, while any iron oxide present in the feed materialwill not go into solution. Other solutions, such as strong basicsolutions having a pH greater than about 10 or strong acidic solutionshaving a pH less than about 3, also can be used to dissolve the zinc,lead and cadmium species; however, if strong acidic solutions are used,iron oxide will dissolve into the solution, and if strong basicsolutions are used, iron oxide will become gelatinous. The lead andcadmium can be removed from the ammonium chloride solution through anelectrochemical reaction which results in the precipitation of lead andcadmium in elemental form. The difference in solubility between diaminozinc dichloride and zinc oxide in water and in ammonium chloridesolutions allows the selective dissolution of the diamino zincdichloride such that pure zinc oxide can be recovered. This also can beused in the crystallization step to improve the relative amounts ofdiamino zinc dichloride and zinc oxide species form. Significantly, allof the zinc can be recycled so that all of the zinc eventually will beconverted into zinc oxide.

The crystallization step of the present process can be done continuouslyin order to increase the throughput and maximize the zinc oxide yieldafter the washing and drying step.

Optional Preroasting Step for Enhanced Zinc Recovery

The zinc dust obtained from various sources have shown by chemicalanalysis to contain from 20%-25% zinc by weight. X-ray diffractionindicates clearly the existence of certain crystalline phases in thisdust, specifically zinc oxide. The positive identification of the ironphase is complicated by the possible structural types (i.e. spinel typeiron phases showing almost identical diffraction patterns). The zincoxide (as well as smaller concentrations of lead or cadmium oxide) areremoved from the initial dust by dissolution in a concentrated ammoniumchloride solution (23% ammonium chloride).

Filtration and washing of the undissolved species leaves a residualpowder. This powder shows a zinc concentration that is still elevated(i.e., 10-13% by weight), but that is not zinc oxide. X-ray diffractionindicates that all crystalline phases can be identified by spinel typephases. The combination of chemical analysis and x-ray diffractionindicates that this powder is a combination of magnetite (iron oxide:Fe₃ O₄). Both of these phases have very similar spinel type structures.The zinc within the franklinite, (Fe, Mn, Zn)(FeMn)₂ O₄, cannot beremoved by dissolution with ammonium chloride. In addition, no simpleextraction process will remove zinc from this stable oxide phase.Although this compound is very stable to oxidation (all elements in thehighest oxidation state), it is relatively easy to destroy this compoundby reduction at elevated temperatures. The reduction of the franklinitein an atmosphere that cannot readily reduce zinc oxide or allow for therapid oxidation of zinc to zinc oxide following reduction andsubsequently recover the zinc oxide by ammonium chloride extraction orsublimation (the highly volatile zinc oxide will sublime from themixture at relatively low temperatures and recondense at the coldlocations of the roaster). The alternative will be complete reduction ofthe franklinite to zinc metal and removal by distillation or separationof the molten zinc by settling techniques. This can be accomplished byincluding the preroasting step disclosed above.

Iron By-Product Recycle

Iron-rich by-products produced during the recovery process can beprocessed further to obtain an end product which can be recycled backinto the leaching step of the recovery process of the present invention.The iron-rich by-products preferably are reduced to DRI in a reductionfurnace. During the reduction process, exhausts fumes which consistsprimarily of zinc, lead and cadmium are produced in the reductionfurnace.

In accordance with a first embodiment, the DRI is sent to a steel millwhere it is used in the production of steel. The steel productionprocess results in exhaust fumes which are processed through thebaghouse or/and a wet scrubber, either or both of which can be locatedat the steel mill. Fumes processed through the baghouse are filtered,and the captured solid residuum, along with an added amount of EAF dust,is recycled back into the waste materials stream whereby it is returnedto the leaching step of the recovery process. Fumes processed throughthe wet scrubber are scrubbed in a liquid stream and the residualimpurities obtained from the scrubbing process are discharged from thewet scrubber directly into the ammonium chloride solution of theleaching step.

In accordance with a second embodiment, the fumes exhausted from thereduction furnace used to produce the DRI are processed through thebaghouse or/and the wet scrubber. Fumes processed through the baghouseare filtered, and the captured solid residuum is recycled back into thewaste material stream, whereby it is returned to the ammonium chloridesolution of the leaching step. In this embodiment, no EAF dust need beadded in with the solid residuum. Fumes processed through the wetscrubber are scrubbed in a liquid stream and the residual impuritiesobtained from the filtering process are discharged from the wet scrubberdirectly into the ammonium chloride solution of the leaching step.

Therefore, iron-rich products which are produced during the recoveryprocess of the present invention can be further processed to producefumes consisting primarily of zinc, lead and cadmium which are capturedin a baghouse or/and a wet scrubber and recycled back into the ammoniumchloride solution of the leaching step to be used in the recoveryprocess.

It should be noted that the locations of the baghouse and wet scrubberare a matter of design choice, plant efficiency and convenience. Thepresent invention is not limited in this aspect. For example, steelmills are equipped with baghouses and wet scrubbers which can be used inthe recycling process of the present invention. Similarly, the locationsof the baghouse or wet scrubber used to process fumes from the DRIreduction furnace are also a matter of design choice, plant efficiencyand convenience.

The above detailed description of a preferred embodiment is forillustrative purposes only and is not intended to limit the spirit orscope of the invention, or its equivalents, as defined in the appendedclaims.

What is claimed is:
 1. A method for the production of a feedstock whichcomprises usable iron constituents from industrial waste streams whichcomprise iron oxides, zinc, lead, and cadmium, comprising the stepsof:a. combining a first waste material stream which is iron poor andcomprises non-iron constituents with a second waste material streamwhich comprises iron and non-iron constituents to produce a wastematerial combination; b. combining said waste material combination withcarbon and roasting said water material combination at an elevatedtemperature resulting in the reduction of at least a portion of the ironoxides in said waste material combination into direct reduced iron andthe production of exhaust vapors comprising iron oxides, and zinc, lead,and cadmium compounds: c. treating said exhaust vapors with an ammoniumchloride solution at an elevated temperature to form a product solutionwhich comprises dissolved non-iron constituents and an undissolvedprecipitate, whereby the iron oxides in said exhaust vapors will becontained in said undissolved precipitate and will not go into solution;d. separating said product solution from said undissolved precipitate;and e. recycling said undissolved precipitate to step b, resulting inthe production of a feedstock which comprises usable iron constituents.2. The method as claimed in claim 1, wherein said undissolvedprecipitate is roasted at a temperature of between 980° C. and 1315° C.3. The method as claimed in claim 2, wherein the concentration of saidammonium chloride solution is approximately 23% by weight.
 4. The methodas claimed in claim 3, wherein said waste material combination isroasted at a temperature of at least 500° C. in a reducing atmosphere.5. The method as claimed in claim 1, wherein said first waste materialstream is selected from the group consisting of waste streams from saidroasting step, waste streams from ore smelting processes, waste streamsfrom metals making processes, waste streams from metals products makingprocesses, waste streams from iron-making processes, and waste streamsfrom steel-making processes.
 6. The method as claimed in claim 5,wherein said second waste material stream is selected from the groupconsisting of waste streams from metals making processes and wastestreams from metals products making processes.
 7. The method as claimedin claim 5, wherein said first waste material stream are fumescomprising particulate matter.
 8. The method as claimed in claim 7,wherein said fumes are filtered through a baghouse to remove at least aportion of said particulate matter, said removed portion of saidparticulate matter constituting said first waste material stream.
 9. Themethod as claimed in claim 8, wherein said undissolved precipitate isroasted at a temperature of between 980° C. and 1315° C.
 10. The methodas claimed in claim 9, wherein the concentration of said ammoniumchloride solution is approximately 23% by weight.
 11. The method asclaimed in claim 10, wherein said product solution is further treated torecover said dissolved non-iron constituents as chemical values.
 12. Themethod as claimed in claim 11, wherein said product solution containszinc-displaceable metal ions and, further comprising the steps of:f.adding zinc metal to said product solution whereby the zinc-displaceablemetal ions contained within said product solution are displaced by saidzinc metal and precipitate out of said product solution as metals; g.separating said metals from said product solution and lowering thetemperature of said product solution thereby precipitating at least aportion of the zinc component of said product solution as a mixture ofcrystallized zinc compounds; h. separating said crystallized zinccompounds from said product solution and washing said crystallized zinccompounds with a wash water thereby solubilizing certain of said zinccompounds; and i. separating any remaining crystallized zinc compoundsfrom said product solution and drying said remaining crystallized zinccompounds at a temperature of between about 100° C. and 200° C.resulting in the recovery of a zinc oxide product of 99% or greaterpurity.
 13. The method as claimed in claim 1, wherein at least a portionof any iron and non-iron constituents contained in said first wastematerial stream and in said second waste material stream are solids. 14.A method for the production of a feedstock which comprises usable ironconstituents from industrial waste streams, which comprise iron oxides,zinc, lead, and cadmium, emanating from high-temperature processes,comprising the steps of:a. scrubbing a first waste material stream whichis iron poor and comprises non-iron constituents; b. combining thescrubbant with carbon and roasting said scrubbant at an elevatedtemperature resulting in the reduction of at least a portion of the ironoxides in said scrubbant into direct reduced iron and the production ofexhaust vapors comprising zinc, lead, and cadmium compounds; c.combining said exhaust vapors and a second waste material stream whichcomprises iron and non-iron constituents with an ammonium chloridesolution at an elevated temperature to form a product solution whichcomprises dissolved non-iron constituents and an undissolvedprecipitate, whereby any iron oxide in said waste material combinationwill be contained in said undissolved precipitate and will not go intosolution; d. separating said product solution from said undissolvedprecipitate; and e. recycling said undissolved precipitate to step b,resulting in the production of a feedstock which comprises usable ironconstituents.
 15. The method as claimed in claim 14, wherein saidundissolved precipitate is roasted at a temperature of between 980° C.and 1315° C.
 16. The method as claimed in claim 15, wherein theconcentration of said ammonium chloride solution is approximately 23% byweight.
 17. The method as claimed in claim 16, wherein said first wastematerial stream is selected from the group consisting of waste streamsfrom said roasting step, waste streams from ore smelting processes,waste streams from metals making processes, waste streams from metalsproducts making processes, waste streams from iron-making processes, andwaste streams from steel-making processes.
 18. The method as claimed inclaim 17, wherein said second waste material stream is selected from thegroup consisting of waste streams from metals making processes andmetals products making processes.
 19. The method as claimed in claim 17,wherein said first waste material stream are times.
 20. The method asclaimed in claim 14, wherein said first waste material stream isscrubbed using a second ammonium chloride solution.
 21. The method asclaimed in claim 19, wherein said product solution is further treated torecover said dissolved non-iron constituents as chemical values.
 22. Themethod as claimed in claim 21, further comprising the steps of:e. addingzinc metal to said product solution whereby any zinc-displaceable metalions contained within said product solution are displaced by said zincmetal and precipitate out of said product solution as metals; f.separating said metals from said product solution and lowering thetemperature of said product solution thereby precipitating at least aportion of any zinc component of said product solution as a mixture ofcrystallized zinc compounds; g. separating said crystallized zinccompounds from said product solution and washing said crystallized zinccompounds with a wash water thereby solubilizing certain of said zinccompounds; and h. separating any remaining crystallized zinc compoundsfrom said product solution and drying said remaining crystallized zinccompounds at a temperature of between about 100° C. and 200° C.resulting in the recovery of a zinc oxide product of 99% or greaterpurity.
 23. A method for the production of a feedstock which comprisesusable iron constituents from industrial waste streams which compriseiron oxides, zinc, lead, and cadmium, emanating from high-temperatureprocesses, comprising the steps of:a. scrubbing a first waste materialstream which is iron poor and comprises non-iron constituents with anammonium chloride solution; b. combining a second waste material streamwhich comprises iron and non-iron constituents and preroasting saidsecond waste material stream at an elevated temperature resulting in thereduction of at least a portion of the iron oxides in said second wastematerial into direct reduced iron and the production of exhaust vaporscomprising zinc, lead, and cadmium; c. combining said exhaust vaporswith the ammonium chloride scrubbant to form a product solution whichcomprises dissolved non-iron constituents and an undissolvedprecipitate, whereby any iron oxide in said exhaust vapors will becontained in said undissolved precipitate and will not go into solution;d. separating said product solution from said undissolved precipitate;and e. recycling said undissolved precipitate to step b, resulting inthe production of a feedstock which comprises usable iron constituentsin the form of direct reduced iron.
 24. The method as claimed in claim23, wherein said first waste material stream is selected from the groupconsisting of waste streams from said roasting step, waste streams tomore smelting processes, waste streams from metals making processes,waste streams from metals products making processes, waste streams fromiron-making processes, and waste streams from steel-making processes.25. The method as claimed in claim 24, wherein said first waste materialstream are fumes.
 26. The method as claimed in claim 4, wherein saidfirst waste material stream is off-gases produced during said step ofpreroasting.